Wastewater from domestic and industrial sources must be treated before it is reintroduced into the water supply. In this regard, publicly owned treatment works (POTW) use various techniques to purify water, depending on the pollutant content of the water. The diversity of pollutants in wastewater may be great, including such substances as colorants and pigments. Colorants encompass both color and opacity agents, which impart color to water and are exemplified by dyestuffs. Colorants typically have either a negative or neutral electric charge and are water soluble. In contrast, pigments are water insoluble and are exemplified by the coloring agents suspended in paint and ink. However, in this application the term colorant encompasses both color and opacity agent type colorants and pigments.
Typically, POTW use pretreatment techniques in combination with conventional biomass sludge systems and secondary purification systems to treat incoming wastewater from domestic and industrial waste streams. Such pretreatment techniques include the use of chemical oxidizing agents. These oxidizing agents are discussed in the April 1998 article entitled Decolorization of Dyes Using Uv/H.sub.2 O.sub.2 Photochemical Oxidation by Yang, Wyatt, and Baharsky in the AATCC Textile Chemist and Colorist, Vol. 30, No. 4, pp. 27-35. However, colorants in waste streams present a challenge to POTW since they tend to be resistant to biomass degradation as well as oxidative purification. Thus, colorants pass through water treatment systems thus leaving residual color or opacity agents in purified potable water. Concerns over possible carcinogens in the water supply make this undesirable in addition to aesthetic considerations.
The difficulty in decolorizing colorants in wastewater is primarily due to their extreme stability. The few oxidizing agents that are somewhat effective in reducing colorant content, such as chlorine and ozone, suffer from efficiency and cost limitations. These oxidizing agents are expensive and do not significantly reduce the colorant content of wastewater unless added in very substantial quantities. Furthermore, these oxidizing agents have the potential for generating toxic organics that are harmful to both humans and the environment. Other oxidizing agents, such as hydrogen peroxide, which are discussed in the AATCC article, are only effective with limited classes of dyes.
As a result of the difficulties in eliminating colorants from industrial waste streams, those industries and mills that discharge colorants from their manufacturing processes are under increased pressure by regulatory agencies to eliminate these pollutants from their waste streams before these are discharged to the POTW facilities.
As a result of this increased governmental pressure to address the colorant pollution problem at the source, methods have been recently developed that reduce some specific colorants. The current commercial and experimental methods for decolorizing wastewater are discussed in depth in the 1996 publication of John Wiley & Sons, Inc. entitled Environmental Chemistry of Dyes and Pigments, edited by A. Reife and H. Freeman. These methods include carbon adsorption, dye reduction, ozonations, electrochemical techniques, chemical pretreatments, aerobic and anaerobic treatments, and powdered activated carbon-activated sludge system or PACT.RTM.. These removal methods differ in efficiency levels and effectiveness depending on the type of colorant to be removed. Many require relatively lengthy time periods to be effective.
Even with the availability of these specific colorant removal techniques, no single method is known which efficiently and effectively removes colorants derived from most acid dyes, fiber reactive dyes, direct dyes, disperse dyes, basic dyes, vat and sulfur pigments and water-based paint and ink pigments. Although, while adsorption removal methods using activated carbon are widely used for dyes, they are ineffective for removal of disperse dyes and vat pigments. Anionic dyes, such as fiber reactive dyes, require reduction by hydrosulfite before activated carbon adsorption methods will effectively remove colorants.
In many situations, polymer flocculation, in combination with adsorption, is required for effective colorant removal. In others, coagulants such as alum, lime, ferric salts, with or without cationic polymers, or alum with powdered activated carbon (PAC), are used for colorant removal. However, multi-component and multi-stage colorant removal techniques are time consuming and expensive. The efficiency of the multi-stage colorant removal techniques often does not justify their costs.
Thus, given the practical limitations of currently available colorant removal techniques, a need remains for a colorant removal method that allows for the efficient removal of a wide range of colorants over relatively short time periods. Furthermore, a need remains for a colorant removal method that does not require costly steps to achieve a commercially acceptable level of residuals colorants.